The present invention relates to a melt-processable fluorine-containing resin composition, and more particularly to a resin composition comprising not less than 0.01 to less then 500 ppm of a thermal stabilizer and a fluorovinyl ether-tetrafluoroethylene copolymer (hereinafter referred to as "PFA"), which has excellent thermal stability and can provide products which are not colored and have no bubble.
Melt-processable fluorine-containing resins including PFA have melt-flowability, that is, the melt viscosity of the resins is generally lower than 10.sup.6 poises at an optimum processing temperature and, therefore, they provide a film having less pinholes and voids as compared with polytetrafluoroethylene (hereinafter referred to as "PTFE") which is a typical and the most widely used fluorine-containing resin since it has an extremely high melt viscosity from 10.sup.10 to 10.sup.11 poises even at a processing temperature, i.e. about 380.degree. C. and therefore, it has no melt-flowability. Accordingly, the melt-processable fluorine-containing resins are useful for use in coating or lining in the state of powder as well as a dispersion, and the obtained film is excellent in chemical resistance and corrosion resistance.
However, the thermal stability of the melt-processable fluorine-containing resins at high temperatures in the vicinity of their sintering temperatures is inferior to that of PTFE, and this often causes trouble in processing. That is, when the resins are heated at a suitable sintering temperature of 340.degree. to 380.degree. C. for a long time more than 30 minutes, the resins partially cause thermal decomposition, and particularly when the coated film is thick, bubbles are formed in the film inevitably. This phenomenon is accelerated by the influence of oxygen in air.
For this reason, there have hitherto been proposed (1) a process in which thickness of a coating per one application is made as small as possible (about 50 .mu.m) and the coating procedures are repeated, (2) a process in which a resin having a low molecular weight is used and the coating is sintered at a considerably low temperature (320.degree. to 340.degree. C.), (3) a process in which a thermal stabilizer is admixed with a melt-processable fluorine-containing resin in a large amount (for instance, U.S. Pat. No. 4,276,214 and U.S. Pat. No. 4,248,763), and the like.
However, the above process (1) has the disadvantages that the application and sintering procedures must be repeated more than 20 times until a sintered film reaches a desired thickness (generally, not less than 1000 .mu.m) and the processing conditions are narrow. Also, the process (2) results in the formation of bubbles when the thickness of the film per one application exceeds 100 .mu.m, and therefore, it is low in productivity as in the process (1). Further, the obtained film is inferior in properties such as stress crack resistance and solvent crack resistance because of using a low molecular weight resin.
On the other hand, though the process (3) has the effects that the formation of bubbles is inhibited and the processing conditions become wide by blending a thermal stabilizer, for obtaining the above effects, it is required to admix the thermal stabilizer with a melt-processable fluorine-containing resin in an amount of not less than 0.05 part by weight based on 100 parts by weight of the resin (not less than 500 ppm). When, however, using the thermal stabilizer in such a large amount, the obtained product is colored by the remaining thermal stabilizer, thus resulting in the lowering of good value of the product.
An object of the present invention is to solve the above problems and to provide a melt-processable fluorine-containing resin composition having excellent thermal stability, that is, the formation of bubbles is not caused upon sintering and the processing conditions can be made wide, and capable of giving products which are not colored.
This and other objects of the present invention will become apparent from the description thereinafter.